1. Field of the Invention
The present invention generally relates to soil remediation systems and methods. Embodiments of the invention relate to systems and methods of heating contaminated soil at one or more treatment sites.
2. Description of Related Art
Soil contamination is a matter of concern in many locations. “Soil” refers to unconsolidated and consolidated material in the ground. Soil may include natural formation material such as dirt, sand, and rock, as well as fill material. Soil may be contaminated with chemical, biological, and/or radioactive compounds. Contamination of soil may occur in a variety of ways, such as material spillage, leakage from storage vessels, and landfill seepage. Public health concerns may arise if contaminants migrate into aquifers or into air. Soil contaminants may also migrate into the food supply through bioaccumulation in various species in a food chain.
There are many ways to remediate contaminated soil. “Remediating soil” means treating the soil to reduce contaminant levels within the soil or to remove contaminants from the soil. An ex situ method of remediating contaminated soil is to excavate the soil and then process the soil in a separate treatment facility to reduce contaminant levels within the soil or to remove contaminants from the soil. Alternatively, contaminated soil may be remediated in situ.
Thermal desorption is a soil remediation process that may involve in situ or ex situ heating of contaminated soil. Heating the soil may reduce soil contamination by processes including, but not limited to, vaporization and vapor transport of contaminants from the soil, entrainment and removal of contaminants in water vapor and/or an air stream, thermal degradation (e.g., pyrolysis), and/or conversion of contaminants into non-contaminant compounds by oxidation or other chemical reactions within the soil. During thermal remediation, a vacuum may be applied to the soil to remove off-gas from the soil. Vacuum may be applied at a soil/air interface or through collection ports (e.g., vacuum or vapor extraction wells) placed within the soil. The vapors may entrain volatile contaminants and carry these contaminants toward the vacuum source. Vapors removed from the soil by the vacuum may include contaminants from the soil. The vapors may be transported to a treatment facility. The vapors removed from the soil may be processed in the treatment facility to remove contaminants from the vapors or to reduce contaminant levels within the vapors.
Soil may be heated by methods including, but not limited to, radiative heating, conductive heating, radio frequency heating, and/or electrical resistivity heating. For shallow contaminated soil, a thermal blanket placed on top of the soil or heaters placed horizontally in trenches within the contaminated soil may be used to apply heat to the soil. Shallow contaminated soil includes soil contamination that does not extend below a depth of about 1 m to about 2 m. For deeper contaminated soil, heater wells or heater/vapor extraction wells may be used to apply heat to the soil.
A vacuum may be applied to remove vapors from contaminated soil. U.S. Pat. No. 4,984,594 issued to Vinegar et al., which is incorporated by reference as if fully set forth herein, describes an in situ thermal desorption (ISTD) process for soil remediation of low depth soil contamination. U.S. Pat. No. 5,318,116 issued to Vinegar et al., which is incorporated by reference as if fully set forth herein, describes an ISTD process for treating contaminated subsurface soil with conductive heating.
Heat added to contaminated soil may raise a temperature of the soil above vaporization temperatures of soil contaminants. If soil temperature exceeds a vaporization temperature of a soil contaminant, the contaminant may vaporize. A vacuum may be used to draw the vaporized contaminant out of the soil. The presence of water vapor may result in vaporization of less volatile contaminants at or near the boiling point of water. Heating the soil to a temperature below vaporization temperatures of contaminants may also have beneficial effects. Increasing soil temperature may increase a vapor pressure of contaminants in the soil and allow a vacuum system to remove a greater portion of contaminants from the soil than possible at lower soil temperatures. Evaporation of contaminants into air or water vapor streams may be enhanced by heating. Heat applied to the soil may also result in the destruction of contaminants in situ.
U.S. Pat. No. 5,190,405 issued to Vinegar et al., which is incorporated by reference as if fully set forth herein, describes an in situ method for removing soil contaminants using thermal conduction heating and application of a vacuum.
U.S. Pat. No. 5,229,583 issued to van Egmond et al., U.S. Pat. No. 5,233,164 issued to Dicks et al., and U.S. Pat. No. 5,221,827 issued to Marsden et al., all of which are incorporated by reference as if fully set forth herein, describe surface heating soil remediation systems.
U.S. patent application Ser. No. 09/549,902 of Vinegar et al. and U.S. patent application Ser. No. 09/836,447 of Vinegar et al., both of which are incorporated by reference as if fully set forth herein, describe heater elements placed horizontally within trenches in the soil for remediation.
U.S. Pat. No. 5,553,189 issued to Stegemeier et al., which is incorporated by reference as if fully set forth herein, describes a shallow pit for remediating near surface soil contamination.
U.S. Pat. No. 5,249,368 issued to Bertino et al., which is incorporated by reference as if fully set forth herein, describes a sealed roll-off container for contaminated soil.
A soil remediation system may include four major systems. The systems may be a heating and vapor extraction system, an off-gas collection piping system, an off-gas treatment system, and instrumentation and power control systems.
A heating and vapor extraction system may be formed of wells inserted into the soil for deep soil contamination or of thermal blankets for shallow soil contamination. A combination of wells and thermal blankets may also be used. For example, thermal blankets may be placed at centroids of groups of wells. The thermal blankets may inhibit condensation of contaminants near the soil surface. Soil may be heated by a variety of methods. Methods for heating soil include, but are not limited to, heating substantially by thermal conduction, heating by radio frequency heating, or heating by electrical soil resistivity heating. Thermal conductive heating may be advantageous because temperature obtainable by thermal conductive heating is not dependent on an amount of water or other polar substance in the soil. Soil temperatures substantially above the boiling point of water may be obtained using thermal conductive heating. Soil temperatures of about 100° C., 200° C., 300° C., 400° C., 500° C. or greater may be obtained using thermal conductive heating.
Wells may be used to supply heat to the soil and to remove vapor from the soil. The term “wells” refers to heater wells, vapor extraction wells, and/or combination heater/vapor extraction wells. Heater wells supply thermal energy to the soil. Vapor extraction wells may be used to remove off-gas from the soil. Vapor extraction wells may be connected to an off-gas collection piping system. A vapor extraction well may be coupled to a heater well to form a heater/vapor extraction well. In a region adjacent to a heater/vapor extraction well, air and vapor flow within the soil may be counter-current to heat flow through the soil. The heat flow may produce a temperature gradient within the soil. The counter-current heat transfer relative to mass transfer may expose air and vapor that is drawn to a vacuum source to high temperatures as the air and vapor approaches and enters the heater/vapor extraction well. A significant portion of contaminants within the air and vapor may be destroyed by pyrolysis and/or oxidation when the air and vapor passes through high temperature zones surrounding and in heater/vapor extraction wells. In some soil remediation systems, only selected wells may be heater/vapor extraction wells. In some soil remediation systems, heater wells may be separate from the vapor extraction wells. In some embodiments, heaters within heater wells and within heater/vapor extraction wells may operate in a range from about 650° C. to about 870° C.
Thermal conductive heating of soil may include radiatively heating a well casing, which conductively heats the surrounding soil. Coincident or separate source vacuum may be applied to remove vapors from the soil. Vapor may be removed from the soil through extraction wells. U.S. Pat. No. 5,318,116 issued to Vinegar et al., which is incorporated by reference as if fully set forth herein, describe ISTD processes for treating contaminated subsurface soil with thermal conductive heating applied to soil from a radiantly heated casing. The heater elements are commercial nichrome/magnesium oxide tubular heaters with Inconel 601 sheaths operated at temperatures up to about 1250° C. Alternatively, silicon carbide or lanthanum chromate “glow-bar” heater elements, carbon electrodes, or tungsten/quartz heaters could be used for still higher temperatures. The heater elements may be tied to a support member by banding straps.
Wells may be arranged in a number of rows and columns. Wells may be staggered so that the wells are in a triangular pattern. Alternatively, the wells may be aligned in a rectangular pattern, pentagonal pattern, hexagonal pattern or higher order polygonal pattern. In certain well pattern embodiments, a length between adjacent wells is a fixed distance so that a polygonal well pattern is a regular well pattern, such as an equilateral triangle well pattern or a square well pattern. In other well pattern embodiments, spacing of the wells may result in non-regular polygonal well patterns. A spacing distance between two adjacent wells may range from about 1 m to about 13 m or more. A typical spacing distance may be from about 2 m to about 4 m.
Wells inserted into soil may be extraction wells, injection wells and/or test wells. An extraction well may be used to remove off-gas from the soil. The extraction well may include a perforated casing that allows off-gas to pass from the soil into the extraction well. The perforations in the casing may be, but are not limited to, holes and/or slots. The perforations may be screened. The casing may have several perforated zones at different positions along a length of the casing. When the casing is inserted into the soil, the perforated zones may be located adjacent to contaminated layers of soil. The areas adjacent to perforated sections of a casing may be packed with gravel or sand. The casing may be sealed to the soil adjacent to non-producing layers to inhibit migration of contaminants into uncontaminated soil. An extraction well may include a heating element that allows heat to be transferred to soil adjacent to the well.
In some soil remediation processes, a fluid may be introduced into the soil. The fluid may be, but is not limited to, a heat source such as steam, a solvent, a chemical reactant such as an oxidant, or a biological treatment carrier. A fluid, which may be a liquid or gas, may be introduced into the soil through an injection well. The injection well may include a perforated casing. The injection well may be similar to an extraction well except that fluid is inserted into the soil through perforations in the well casing instead of being removed from the soil through perforations in the well casing.
A well may also be a test well. A test well may be used as a gas sampling well to determine location and concentration of contaminants within the soil. A test well may be used as a logging observation well. A test well may be an uncased opening, a cased opening, a perforated casing, or combination cased and uncased opening.
A wellbore for an extraction well, injection well, or test well may be formed by augering a hole into the soil. Cuttings made during the formation of the augered hole may have to be treated separately from the remaining soil. Alternatively, a wellbore for an extraction well, injection well, or test well may be formed by driving and/or vibrating a casing or insertion conduit into the soil. U.S. Pat. No. 3,684,037 issued to Bodine and U.S. Pat. No. 6,039,508 issued to White describe devices for sonically drilling wells. Both of these patents are incorporated by reference as if fully set forth herein.
A covering may be placed over a treatment area. The covering may inhibit fluid loss from the soil to the atmosphere, heat loss to the atmosphere, and fluid entry into the soil. Heat and vacuum may be applied to the cover. The heat may inhibit condensation of contaminants on the covering and in soil adjacent to the covering. The vacuum may remove vaporized contaminants from the soil adjacent to a soil/air interface to an off-gas treatment system.
An off-gas collection piping system may be connected to vapor extraction wells of a heating and vapor extraction system. The off-gas collection piping system may also be connected to an off-gas treatment system so that off-gas removed from the soil may be transported to the treatment system. Typical off-gas collection piping systems are made of metal pipe. The off-gas collection piping may be un-heated piping that conducts off-gas and condensate to the treatment facility. Alternatively, the off-gas collection piping may be heated piping that inhibits condensation of off-gas within the collection piping. The use of metal pipe may make a cost of a collection system expensive. Installation of a metal pipe collection system may be labor and time intensive. In some embodiments, off-gas collection piping may be or may include polymer piping and/or flexible hose.
Off-gas within a collection piping system may be transported to an off-gas treatment system. The treatment system may include a vacuum system that draws off-gas from the soil. The treatment system may also remove contamination within the off-gas to acceptable levels. The treatment facility may include a reactor system, such as a thermal oxidizer, to eliminate contaminants or to reduce contaminants within the off-gas to acceptable levels. Alternatively, the treatment system may use a mass transfer system, such as passing the off-gas through activated carbon beds, to eliminate contaminants or to reduce contaminants within the off-gas to acceptable levels. A combination of a reactor system and a mass transfer system may also be used to eliminate contaminants or to reduce contaminants within the off-gas to acceptable levels.
Instrumentation and power control systems may be used to monitor and control the heating rate of a soil remediation system. The instrumentation and power control system may also be used to monitor the vacuum applied to the soil and to control of the operation of the off-gas treatment system. Electrical heaters may require controllers that inhibit the heaters from overheating. The type of controller may be dependent on the type of electricity used to power the heaters. For example, a silicon controlled rectifier may be used to control power applied to a heater that uses a direct current power source, and a zero crossover electrical heater firing controller may be used to control power applied to a heater that uses an alternating current power source. In some embodiments, the use of controllers may not be necessary.
A barrier may be placed around a region of soil that is to be treated. The barrier may include metal plates that are driven into the soil around a perimeter of a contaminated soil region. A top cover for the soil remediation system may be sealed to the barrier. The barrier may limit the amount of air and water drawn into the treatment area from the surroundings. The barrier may also inhibit potential spreading of contamination from the contaminated region to adjacent areas and/or the atmosphere.